Physical exertion on Lake Titicaca, more than 4000 metres up, is easier for the Bolivian locals.© Digital Vision

Inhabitants of the harsh, high-altitude plateaus of the world have evolved to survive lofty conditions in different ways, a new genetic study reveals.

Cynthia Beall of Case Western Reserve University in Cleveland, Ohio, studies the inhabitants of the Tibetan plateau, the Ethiopian plateau and the Andean Altiplano in Peru and Bolivia - locations where hardy residents routinely dwell at altitudes of more than 4,000 metres.

At these altitudes, the thin air leaves most tourists gasping for breath. "You're getting one third less oxygen every time you take a lungful of air," says Beall. Yet the regular inhabitants appear almost superhuman in their ability to function normally in such extreme conditions.

Previous studies have shown that the Tibetan, Ethiopian and Andean populations have developed slightly different ways of boosting their oxygen levels to cope with the thin air. Those in the Andes pump out more haemoglobin - a molecule that carries oxygen around in the blood. The Tibetans, by contrast, have relatively low haemoglobin levels but breathe faster to take in more oxygen. "The slightest bit of exercise makes them really pant," Beall says.

To understand what underlies these physiological changes, Beall has begun to explore each population's genetic code: specifically, a short region of DNA in the cells' power-generating mitochondria. This DNA is expected to contain distinctive sequences that might explain how cells churn out energy with little oxygen to fuel them.

Beall has examined this DNA from several hundred people in populations from the Andes, Tibet and Ethiopia. Their genetic sequences are largely dissimilar, she revealed at a meeting of the American Association for the Advancement of Science in Seattle this week. This supports the idea that the three groups have separately evolved different tactics to survive in the thin air.

Altitude problem

The analysis also reveals something about how the ancestors of these populations spread onto the high plateaux, says Beall. If, for example, a population is descended from just a few pioneers, she would expect that population to have similar genetic code today.

Instead, Beall found that the genetic sequences of individuals within each population differed quite a lot from one another. This suggests that a relatively large group of people took to the high ground long ago.

The genetic findings also fit with archaeological data showing that the Tibetan, Ethiopian and Andean highlands were first populated at different times in history, which means their inhabitants have had different amounts of time to adapt to the problems posed by the harsh climes.

The Ethiopians are thought to have reached the plateau perhaps 50,000 years ago, the Tibetan plateau was populated about 23,000 years ago, and the Andean Altiplano some 10,000 years ago - after humans arrived in the New World.

Perhaps all three communities are at various stages of the same path of evolutionary development, says Beall. Given enough time, they might all evolve the same adaptations to the high life. But it's perhaps more likely, she says, that they are all on their own genetic paths towards different adaptations.